Synthetic gas, mainly containing hydrogen and carbon monoxide both obtained from natural gas containing methane as main component, has been an important industrial material and used as raw material for methanol production, ammonia production, oxo synthesis, or the like. In production of hydrocarbon fuel, such as GTL (gas to liquids) and DME (dimethyl ether) synthesis which have recently drawn attention as environment-friendly fuel, a common production method includes steps of once converting natural gas as raw material into such synthetic gas and performing GTL or the synthesis of DME.
There are various methods for producing synthetic gas from natural gas, and four basic methods include a steam reforming process, a partial combustion process, an autothermal reforming, and a catalytic partial oxidation process. Among these, the catalytic partial oxidation process, which is related to the present invention, is a process in which a hydrocarbon material (e.g., natural gas), steam, and oxygen-containing gas (e.g., air and pure oxygen) are supplied to a reaction container having a catalyst therein, and a partial oxidation reaction of hydrocarbon is advanced in the presence of the catalyst. As one example of the catalytic reactor for such a partial oxidation reaction, there can be mentioned a vertical reaction tube disclosed in U.S. Pat. No. 5,112,527.
In addition, for the catalytic partial oxidation process, for example, there has been known a process in which natural gas, steam and oxygen are supplied to a reaction container having a ruthenium catalyst therein, and the partial oxidation reaction is allowed to proceed in the presence of the ruthenium catalyst to thereby obtain synthetic gas (see, for example, Japanese Patent Unexamined Application Publication No. 2007-69151).
A partial oxidation reaction of methane in the presence of the catalyst advances chiefly with the following reactions (1)-(4):CH4+½O2→2H2+CO  (1)CH4+2O2→CO2+2H2O  (2)CO+H2O<-->CO2+H2  (3)CH4+H2O<-->CO+3H2  (4)
Until oxygen fed from a reaction container inlet is used up, the reactions (1) and (2) are predominant, and after the oxygen has run out, the equilibrium reactions (3) and (4) proceed. In the catalytic partial oxidation reaction, the reactions (1) and (2) may occur in a gas phase, since a reaction gas (material fluid) to be introduced to the catalyst is a premixed gas containing the hydrocarbon and the oxygen. In a case where the reactions (1) and (2) occur in the gas phase before the reaction gas reaches the catalyst, troubles arise such as carbon precipitation and backfire, and thus after mixing the hydrocarbon and the oxygen, it is notably important to carefully operate the reactor in such a manner that the reactions (1) and (2) do not occur in the gas phase before the reaction gas reaches the catalyst.
As catalyst to be used in such a reaction system, a ruthenium catalyst has been known. It has been known that, in a completely oxidative atmosphere, ruthenium in the ruthenium catalyst is oxidized to ruthenium oxide. Representative forms of ruthenium oxide include RuO2, RuO3 and RuO4. From among these, RuO3 and RuO4 are volatile and likely to scatter. Therefore, in a case where the partial oxidation reaction is performed using the ruthenium catalyst, attention should be paid to an operation condition so as to prevent scattering which may be caused by excessive oxidation of ruthenium.